Did you ever get this resolved? I am having very similar issues where the 3rd party jar file seems invisible no matter where I put it.
1. If the codebase is ".", then the archives are taken from the root of ur particular deployment, i.e. xxx.sar/<archive-path>
2) If the codebase is not ".", then the path is taken from the server.home.url which is typically jboss_home/server/default.
so if you put ur entry as
<classpath codebase="lib" archives="xxx.jar,yyy.jar", you should typically be putting it in the jboss_home/server/default/lib directory. As far as the MBean itself is concerned, i think it looks it the jar in the SAR for the MBean but not for external jar files.